A GSS-based Server Design in .NET framework

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20.3 A GSS-based Server Design
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In this section, we apply the well-known Grouped Sweeping Scheme (GSS) scheduler for use in a SS-VoD server. This design will serve as a baseline to compare the ef cient server design to be presented in Section 20.4. Let N be the number of disks in the system, assuming the disks are homogeneous. The disk s storage is divided into xed-size blocks of Q bytes each, and a service group is de ned to consist of all the data blocks at the same location from each of the N disks. Video data are striped across the N disks as shown in Figure 20.1, effectively forming a RAID-4 [10] disk array without parity. Denote the jth data block of video i by bi, j . Then, the rst N blocks of video i, [bi, 1 , bi, 2 , bi, 3 . . . bi, N ] are allocated to the rst service group. This storage allocation scheme ensures load balance among all N disks. Figure 20.2 depicts the Grouped Sweeping Scheme (GSS). In GSS, a macro-round is divided evenly into G micro-rounds, with each micro-round serving a separate group of video streams. Assuming all the videos are encoded using constant-bit-rate (CBR) encoding method with the same bit-rate RV , then in each micro-round the server retrieves one data block from each disk for each channel, and this data block is then multicast over the next G micro-rounds (i.e., one
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bi,1 bi,N i,N+1 bi, +1 i,2N+1
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bi,2 bi, 2 i,N+2 bi,2N+2 i,2N+2
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bi,,3 i,3 bi, 3 i,N+3 bi, +3 i,2N+3
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Disk 1 Disk 2
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Figure 20.1 Allocation of video blocks among disks for video i
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Disk 3
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Disk N 2 Disk N 1 Disk N
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Scalable Continuous Media Streaming Systems
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TR static channel 1 static channel 2 static channel 3 Time
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Micro-round Mi round
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All channels in the same group start transmit at the same time A macro round Transmission Static round Data Retrieval Micro-round Data for all the requests in a group Dynamic round Static round
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Figure 20.2 Transmission and retrieval schedule of GSS
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macro-round). Reducing G we can pack more video streams in a group and this results in increased disk ef ciency, albeit at the expense of increased buffer requirement and scheduling delay and vice versa. In the extreme case with G = 1, GSS reduces to SCAN; and in the other extreme case with G = n, where n is the maximum number of streams that the system can support, GSS reduces to rst-come- rst-serve. In SS-VoD, static channel starts once every TR seconds, where TR is given by TR = L NS (20.1)
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and L is the length of the video title in seconds. However, transmission in GSS can only start at the beginning of a micro-round and hence does not necessary match the transmission schedule of the static channels. In particular, when TR is not divisible by the micro-round time, some of the static channels will not be able to transmit precisely at the scheduled time. To avoid this problem, we can use additional buffers to perform read-ahead to absorb the time differences. However, this will increase the buffer requirement up to 50% and is thus not desirable. Alternatively, we can choose the value of G and Q so that TR is an integer multiple of the duration of a micro-round to avoid the additional buffer requirement. For each video stream, N data blocks, one from each disk, are retrieved in a micro-round for transmission over the next G micro-rounds. Thus, the service round length Tr is given by Tr = NQ , RV (20.2)
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and the total server buffer requirement [6] is given by 1 ), (20.3) G where C is the total number of multicast channels in the server. As the static channels offset TR is integer multiples of the micro-round length, it is easy to see that the static channels will be equally distributed to all G groups. The remaining disk capacity is then used to support dynamic channels. BServer = CNQ(1 +